This application relates to a spark plug for use in conjunction with an internal combustion engine, and, more particularly, to a spark plug having a structure providing improved ignition capability.
Conventional spark plugs for use in internal combustion engines generally include a tube-shaped metallic shell, an insulator, a center electrode, and a ground electrode. The metal shell has a threaded portion for fitting the spark plug into a combustion chamber of the engine. The insulator has a center bore formed therein and is fixed in the metal shell such that an end of the insulator protrudes from an end of the metal shell. The center electrode is secured in the center bore of the insulator so that an end thereof protrudes from the end of the insulator. The ground electrode has a tip portion and is joined to the end of the metal shell such that the tip portion faces the end of the center electrode through a spark gap therebetween.
In recent years, the demand for internal combustion engines that provide higher power output has led to an increase in the number and/or size of engine intake and exhaust valves in engines, as well as the introduction of water jackets secured to engines to provide for cooling. This has led to a decrease in the amount of space available for spark plug installation in the engine, thereby necessitating the development of spark plugs having a compact structure. More specifically, narrow spark plugs in which the threaded portion of the metal shell has an outer diameter of 12 mm or less are now being standardized. In practice, compact spark plugs with a shell outer diameter of 12 mm or less result in a reduced distance between the metal shell and the center electrode of the insulator. Thus, the volume of the air pocket is accordingly reduced.
During operation of compact spark plugs having a reduced air pocket volume, there is an increased tendency for the spark, rather than forming and remaining at the electrode gap as intended, to creep sideways from the center electrode along the outer surface of the insulator and jump across the air pocket to the metal shell. This phenomena, known as an inside spark or a side fire, can cause a misfire or a partial burning that reduces engine efficiency. Moreover, certain spark plugs, in particular those having a shell thread size of 12 mm or less along with a shell thread reach of 19 mm or more, tend to possess the additional drawback of having a lower resistance to over-torquing forces that can cause the seal between the metal shell and the insulator to loosen from extension.
Accordingly, it is desirable to provide an improved spark plug structure that prevents the inside spark/side fire phenomena and results in a seal that is more resistant to over-torquing forces.